Firmly demonstrate (5)
I believe the answer is:
prove
(Other definitions for prove that I've seen before include "Show to be true" , "Ascertain" , "Demonstrate beyond doubt" , "Establish the validity of" , "13 [SHOW]" .)
I believe the answer is:
prove
(Other definitions for prove that I've seen before include "Show to be true" , "Ascertain" , "Demonstrate beyond doubt" , "Establish the validity of" , "13 [SHOW]" .)